A comparison of two convergent routes for the preparation of metalloporphyrin-core dendrimers: Direct condensation vs. chemical modification

Article abstract of DOI:10.1039/a705410f

Porphyrin-core dendrimers consisting of benzyl ether dendrons assembled around a porphyrin core have been prepared by two different convergent syntheses. The first involves the direct condensation of convergent dendrons having 3,5-disubstituted benzaldehy

Full text of DOI:10.1039/a705410f

J O U R N A L O F
A comparison of two convergent routes for the preparation of
metalloporphyrin-core dendrimers: direct condensation
vs. chemical modification†
´
Keith W. Pollak,a Elizabeth M. Sanfordb and Jean M. J. Frechet*a
C H E M I S T R Y
aDepartment of Chemistry, University of California, Berkeley, CA 94720–1460 USA
bHope College, Holland, Michigan 49422-9000, USA
Porphyrin-core dendrimers consisting of benzyl ether dendrons assembled around a porphyrin core have been prepared by two
different convergent syntheses. The first involves the direct condensation of convergent dendrons having 3,5-disubstituted
benzaldehyde focal points with an equivalent amount of pyrrole. This route which requires very mild conditions is especially useful
for the rapid assembly of small dendrimers but suffers from steric limitations as the size of the dendrons increases above the fourth
generation. The second route involves the attachment of pre-formed benzyl bromide dendrons to a functionalized porphyrin
through a simple Williamson synthesis. This route is also very practical but it requires careful purification of the final product from
the partially functionalized porphyrin dendrimers that are also obtained. MALDI mass spectrometry proved to be a very useful
tool both for monitoring the formation of the dendritic porphyrins and for their characterization.
Metalloporphyrins with specific architecture have been devel-
oped to model various biological systems; two of the common
natural archetypes are chlorophyll, and heme proteins. The
light-harvesting and electron-transfer properties of chlorophyll
have been modelled by synthetic metalloporphyrins with sub-
stituents which modify the photochemical behavior of the
porphyrin.1 Other metalloporphyrins have been synthesized
with either one or both faces of the porphyrin sterically
obstructed to model proteins like hemoglobin and myoglobin
which bind and transport molecular oxygen.2 But the most
common purpose for synthesizing porphyrins with defined
architecture is to model natural oxidation catalysts like
cytochromes.3,4
Some of the latest biological models embed a porphyrin at
the core of a dendrimer. The synthesis of such a macromolecule
could follow either the divergent5 or convergent6 approach to
dendrimers, and indeed both routes have been used by different
research groups.7–10 For example Diederich and co-workers7
have used the divergent approach to grow a polyamide den-
drimer from a porphyrin core. Although this method has been
successful for the preparation of large porphyrin-core dendri-
mers, the sensitivity of the porphyrin moiety must be con-
sidered in the growth of the dendrimers, a process that typically
involves multiple preparative steps and demanding purification
procedures. Such potential problems could be avoided by
incorporating the porphyrin core into the macromolecule at
the last step of the synthesis,8 as is common practice in the
synthesis of dendrimers through the convergent approach.6
A convenient route for the synthesis of porphyrin-core
dendrimers involves the attachment of convergent dendrons
to a pre-formed porphyrin core. Aida and co-workers8 have
used the convergent approach to prepare polyether dendrons
and then attached the dendrons to a functionalized tetraaryl
porphyrin, tetrakis(3,5-dihydroxyphenyl)porphyrin, through a
Williamson ether synthesis. Similarly, Moore et al. attached
polyester dendrons, prepared through the convergent
approach, to a metalloporphyrin core using dicyclohexycar-
bodiimide (DCC) coupling.10 A less obvious convergent route
to porphyrin-core dendrimers might involve the preparation
of dendrons with an aldehyde functionality at their focal point,
and their subsequent assembly into a porphyrin by Lindsey12
condensation with pyrrole. Until now, this in situ assembly of
porphyrin-core dendrimers had not been reported.
In view of the potential application of site-isolated porphyrin
nuclei for electron transfer or other catalytic processes, this
study explores and compares the synthesis of metalloporphy-
rin-core dendrimers of generations 1–4 via two routes utilizing
the convergent growth approach. Route I, in which dendritic
aldehyde and pyrrole are combined in a Lindsey porphyrin
synthesis to generate the porphyrin core in situ (Scheme 1) and
route II, in which zinc tetrakis(3,5-dihydroxyphenyl)porphy-
rin, a porphyrin core with eight reactive phenolic sites,8 is
alkylated with a dendritic bromide in a Williamson ether
synthesis (Scheme 2).
Results and Discussion
Direct formation of the porphyrin core from a dendritic
aldehyde and pyrrole
Benzyl ether dendrons were prepared through the iterative
bromination and alkylation steps described by Hawker and
Fre´chet.6a,11 To prepare the desired generation of dendritic
aldehyde, dendritic bromide was used to alkylate 3,5-
dihydroxybenzaldehyde (Scheme 3). Classical Lindsey conden-
sation12 of the dendritic aldehyde and an equivalent amount
of pyrrole at a concentration of 10−2  in chloroform was
done at room temperature in the presence of a catalytic amount
of trifluoroacetic acid, under very mild conditions that can be
tolerated by various functionalities which might be present in
the dendrons. Monitoring by TLC proved to be effective
because the starting materials, desired product, and polypyrryl-
methane by-products exhibit very different R values. However,
analysis by UV–VIS spectroscopy allowed for a more accurate
f
evaluation of the progress of the condensation because the
desired product absorbs at sharp, characteristic wavelengths:
a strong Soret band at 424 nm and four weak additional bands
at 510, 550, 590 and 650 nm while the polypyrrylmethane by-
products have a broader absorption (Fig. 1). When the relative
intensities of these bands stabilize, the condensation is at
equilibrium. As the generation number is increased the reaction
became more sluggish and reaction times increased from 1.5 h
† Presented at the Third Internation Conference on Materials
Chemistry, MC3, University of Exeter, Exeter, July 21–25 1997.
* E-mail: Frechet@cchem.berkeley.edu
J. Mater. Chem., 1998, 8(3), 519–527
519

for generations 1 and 2, to 10 and 36 h for generation 3 and
4, respectively, though the latter required different reaction
conditions. For the first three generations, the porphyrinogen
condensation product was oxidized directly after condensation
and in the same reaction flask to the free-base porphyrin-core
dendrimers 3a–c in 25–30% yield by heating it to reflux in the
presence of chloranil. Preparation of the fourth generation
free-base porphyrin-core dendrimer 3d was attempted using
these reagents and conditions, but it failed. In the condensation
step, formation of porphyrin was observed in the reaction
samples both by TLC and UV–VIS spectroscopy, yet after the
oxidation step, no porphyrin was produced. This was probably
due to in situ decomposition of the product as a result of the
elevated reaction temperature required for the oxidation reac-
tion using chloranil. Using a stronger oxidizing agent, 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), the oxidation
could be performed at room temperature, thereby preserving
the porphyrinogen and producing 3d in 14% yield. Purification
of all of the free-base porphyrin-core dendrimers by column
chromatography proved to be very simple because the desired
product elutes much faster than the polypyrrylmethane by-
products.
NMR was facilitated by the high degree of symmetry in the
macromolecule. Evidence for the free-base porphyrin core of
3a–d is provided by the eight b-pyrrole and two free-base
protons seen as singlets at 8.9 and −2.9 ppm, respectively. The
spectral features of the dendrons, which surround the porphyrin
core with generational tiers of benzyl ethers, are also readily
attributed. The phenyl protons located on the outermost
phenyl rings are seen at 7.2–7.5 ppm, and those on the aromatic
ring at the meso position of the porphyrin appear as a one-
proton triplet at 7.10 ppm and a two-proton doublet at
7.50 ppm. The benzyl protons appear as a series of singlets
around 4.6–5.0 ppm, depending on the generation number.
Not all of the 13C NMR resonances for the dendrons can be
assigned because of the many overlapping signals,11 and at
high generation, the intensities of some of the resonances of
the core carbons are too weak to be seen. However when the
core carbons are detectable, the porphyrin gives rise to three
distinct resonances:13 C at 145.8 ppm, C at 130.6 ppm, and
a
b
C
at 119.6 ppm. Analysis of the free-base porphyrin-core
meso
dendrimers 3a–d by matrix-assisted laser-desorption ionization
(MALDI) mass spectrometry (Fig. 2) shows that well defined
macromolecules exhibiting the expected peaks corresponding
to M+H+, M+Na+ and/or M+K+ are obtained.
Analysis of the free-base porphyrin core dendrimers by 1H
520
J. Mater. Chem., 1998, 8(3), 519–527

Scheme 1 Reagents and conditions: i, CF CO H; ii, DDQ; iii, Zn(OAc)
3
3
2
J. Mater. Chem., 1998, 8(3), 519–527
521

Scheme 2 Reagents and conditions: i, K CO , 18-crown-6
2
3
522
J. Mater. Chem., 1998, 8(3), 519–527

Fig. 1 (a) UV–VIS spectrum of the crude reaction mixture obtained in
the preparation of a generation 3 porphyrin-core dendrimer 3c via
Route I. (b) UV–VIS absorption spectrum of pure 3c.
phyrin 6 is obtained. Metallation of this porphyrin with zinc
acetate produced zinc 5,10,15,20-tetrakis(3,5-dihydroxyphe-
nyl)porphyrin 4, the octafunctional core for the porphyrin-
core dendrimers. The dendrons used for the subsequent alky-
lation are readily obtained through the iterative bromination
and alkylation steps described by Hawker and Fre´chet.11
Formation of the porphyrin-core dendrimers requires that all
eight phenolic pendant groups of the core be alkylated. This
is best done using a 20% excess of the benzylic bromide
dendron while monitoring the progress of the reaction. Thin
layer chromatography (TLC) is not sufficient for this purpose
because some of the partially alkylated cores such as those
Scheme 3 Reagents and conditions: i, K CO , 18-crown-6
2
3
The free-base porphyrin-core dendrimers 3a–d were quanti-
tatively metallated by dissolving the macromolecule and zinc
acetate in methanol–chloroform (151) and heating at reflux
overnight. Spectroscopic confirmation of the metallation reac-
tion is provided by the UV–VIS absorption spectra since the
metallated dendrimers 1a–d exhibit a strong Soret band at
430 nm and two additional bands at 560 and 600 nm.14 In
addition, the 1H NMR spectra of the products no longer
exhibit the characteristic signal for the two protons located
inside the porphyrin ring of the free-base starting materials.
with six or seven dendrons added to the core exhibit an R
value similar to that of the fully alkylated product. In the final
f
stages of the reaction, monitoring by MALDI mass spec-
troscopy is more effective as the partially alkylated cores are
readily identified as shown in Fig. 3.
Careful control of the reaction temperature is required
during the alkylation as temperature affects both the rate and
the site of alkylation. If the temperature is significantly higher
than 60 °C, the reaction rate increases but significant C-
alkylation involving the 2 position of the phenyl ring of the
porphyrin core is observed as evidenced by the presence of
two singlets of equal intensity at 7.82 and 8.59 ppm character-
istic of the two remaining phenyl protons in the 1H NMR
spectrum of the product mixture. Below 60 °C, the extent of
C-alkylation is insignificant but the reaction times increase
considerably. A reaction temperature of 60 °C produced a good
compromise affording primarily O-alkylated product in a
reasonable reaction time. As expected in view of increased
steric requirements around the core, the reaction time required
for complete alkylation varies as a function of generation: 1, 3
and 5 d for generation 2, 3 and 4, respectively. Regardless of
conditions used the final product must be purified by column
chromatography to remove by-products or partially alkylated
products. Purification is best achieved by chromatography
through silica, eluting with a continuous linear gradient from
hexane to CH Cl . Isolation of the desired product by column
Preparation by dendron alkylation of a pre-formed porphyrin
core
As will be seen below, a comparison of this approach first
reported by Aida and co-workers8 with the direct Lindsey-
type synthesis shows that each route has its own advantages
and shortcomings.
Specifically, this route requires that a porphyrin core with
multiple reactive functionalities be prepared first then coupled
to the appropriate number of benzyl ether dendrons. For
example, Scheme 4 shows the preparation of a core with eight
protected phenolic functionalities by condensation of pyrrole
and 3,5-dimethoxybenzaldehyde. The resulting porphyrinogen
is then oxidized to the free base 5,10,15,20-tetrakis(3,5-dimeth-
oxyphenyl)porphyrin 5 in 52% yield. Following removal of
the methoxy protecting groups by reaction with boron tribrom-
ide, the free base 5,10,15,20-tetrakis(3,5-dihydroxyphenyl)por-
2
2
J. Mater. Chem., 1998, 8(3), 519–527
523

Fig. 2 MALDI mass spectra of the generation 1–4 free-base porphyrin-core dendrimers prepared via Route I. Sodium and potassium adducts
are clearly seen at the right of the main peaks. (a) Generation 1, (b) 2, (c) 3 and (d) 4.
Fig. 3 MALDI mass spectrum of the crude product obtained in the
preparation of fourth generation porphyrin-core dendrimer 1d via
Scheme 4 Reagents and conditions: i, BF ΩOEt ; ii, DDQ; iii, BB ;
Route II. Partly alkylated dendrons are clearly seen at m/z 10 200
3
2
3
iv, Zn(OAc)
and 11 800.
2
chromatography is difficult since the excess dendritic bromide,
some partially alkylated core, and any by-products (i.e. C-
alkylated core) behave very similarly to the desired product
on a silica gel column. However, with careful chromatography,
pure product is isolated, and the excess dendritic bromide can
be recovered. Since high generation dendrons require consider-
able synthetic effort, recovering the unreacted starting material
is often desirable. After evaporating the solvent, the product
was isolated as a violet glass. Subsequent trituration with
methanol allowed the porphyrin-core dendrimer to be handled
524
J. Mater. Chem., 1998, 8(3), 519–527

as a powder. Dendrimers 1b, 1c, and 1d were prepared by this
route in 71, 68 and 20% yields respectively.
inside the porphyrin ring, and specific sites of the porphyrin
ring are designated by a, b and meso.
General procedure for the preparation of dendritic benzyl
bromides11
Conclusions
To a mixture of the appropriate dendritic benzylic alcohol
(1.00 equiv.) and carbon tetrabromide (1.25 equiv.) in the
minimum amount of dry THF was added triphenylphosphine
(1.25 equiv.), and the reaction mixture was stirred under
nitrogen for 20 min. The reaction mixture was then poured
onto water and extracted with CH Cl . The combined organic
This study demonstrates the versatility of the convergent
synthesis for the preparation of dendritic porphyrins. A clear
advantage of the two convergent approaches described herein
is that few steps involving dendrimers are involved; this helps
ensure that impurities or unreacted functionalities do not
accumulate as might be the case in a divergent synthesis
involving multiple steps for the growth of the dendrimer onto
the porphyrin core. This may be particularly useful in instances
where very accurate structural control is required or where
the end-functionalities of the porphyrin ring itself might not
survive the reactions used during multistep syntheses.
2
2
extracts were dried (MgSO ) and evaporated to dryness. The
crude product was purified by column chromatography.
4
Analyses agreed with those published.11
General procedure for preparation of porphyrin-core dendrimers
via Williamson ether synthesis
While both convergent routes benefit from the architectural
control afforded by the convergent synthesis, each route offers
unique synthetic advantages. The direct Lindsey-type conden-
sation is done in the presence of a catalytic amount of acid,
while the alkylation route is done under prolonged basic
conditions. As a result, the choice of a specific route may be
determined by any sensitive functionality that may be present
on the dendrons. A clear advantage of the Lindsey-type route
is the ease of both monitoring of product formation and
purification of the desired free-base dendritic porphyrins by
simple column chromatography. Reaction times for the direct
condensation are also consistently shorter than those for route
II. However, despite the larger number of steps required for
the chemical modification route it affords higher yields than
the direct Lindsey-type synthesis, especially in the preparation
of low generation porphyrin-core dendrimers. The chemical
modification route appears to be more suitable than the
Lindsey route for very large dendrimers since it does not
appear to be as sensitive to steric constraints. It is anticipated
that similar trends would be observed for syntheses using other
types of dendrons.
The second, third and fourth generation porphyrin-core dendri-
mers were synthesized by coupling zinc tetrakis(3,5-dihydroxy-
phenyl)porphyrin 4 to the appropriate dendritic bromide in a
Williamson ether synthesis. Under nitrogen, zinc tetrakis(3,5-
dihydroxyphenyl)porphyrin (1.00 equiv.) and the dendritic
bromide (9.60 equiv) were dissolved in acetone. To this solution,
K CO (16.0 equiv.) and 18-crown-6 (1.60 equiv) were added,
and the mixture was stirred and warmed to 60 °C. The solvent
was evaporated, and the residual solids were partitioned
between water and CH Cl . The layers were separated, and
2
3
2
2
the aqueous layer was extracted with CH Cl (3×). The solvent
2
2
was evaporated, and the residual solids were adsorbed onto
silica (20 ml), and the crude product was purified by chroma-
tography through a 40 ml silica column eluting with a linear
gradient of solvent from pure hexane to pure CH Cl . The
product fractions were collected, and the solvent was evapor-
ated. The product was dissolved in a minimum amount of
CHCl then precipitated into methanol. The precipitate was
then dissolved in a minimum amount of diethyl acetate and
precipitated into diethyl ether. The product was obtained as a
dark purple powder.
2
2
3
Finally, since the convergent synthesis of the dendritic
porphyrin affords precise control of the dendrimer architecture
it provides the means for incorporating potential prosthetic
functionalities at precise locations within the assembly.
Through careful design, functionalizing the dendrons with
either specific guest-binding sites or rate-enhancing ligands
near the catalytic core could create ‘suprabiotic’4 catalysts.
Zn[G-2] P 1b. This was prepared as above from zinc
tetrakis(3,5-dihydroxyphenyl)porphyrin 4 and [G-1]Br; yield:
71%; UV–VIS l/nm (e): 428 (498 000), 560 (20 000), 600 (7000).
4
d
7.03 (t, 4 H, Ar-H), 6.73 (d, 16 H, Ar-H), 6.56 (t, 8 H, Ar-H),
9.18 (s, 8 H, b-H), 7.48 (d, 8 H, Ar-H), 7.33 (m, 80 H, Ph-H),
H
5.06 (s, 16 H, Bn-H), 4.96 (s, 32 H, Bn-H). d 160.09,
157.71 (Ar C-O); 149.92 (a C); 144.80 (Ar C-porph); 139.15 (Ar
C
C-CH ); 136.63 (Ph C-CH ); 132.10 (b C); 128.46, 127.88,
127.43 (Ph C-H); 120.67 (meso C); 115.02, 106.43, 101.65, 96.66
(Ar C-H); 70.00 (Ar/Ph-CH ). Mass spectrum (MALDI-TOF):
2
2
Experimental
General directions
2
m/z, calc. 3225.09; found 3228.3 (Calc. for C
C 78.95, H 5.38, N 1.74; found C 79.06, H 5.57, N 1.62%).
H
N O Zn;
212 172
4 24
Silica for flash chromatography was Merck Silica Gel 60
(230–400 mesh). Matrix-assisted laser-desorption ionization
time of flight (MALDI-TOF) mass spectroscopy was per-
formed on a Finnigan Lasermat or Perseptive Voyager DE.
The energy source was a 337 nm nitrogen laser. Samples were
prepared as 10−4  solutions in tetrahydrofuran. The matrix
was a solution consisting of 0.2  indoleacrylic acid and
7×10−4  sodium dodecyl sulfate in tetrahydrofuran. Four
microlitres of the sample and 40 microlitres of matrix were
combined and analyzed. All NMR spectra were recorded as
Zn[G-3] P 1c. This was prepared as above from zinc
tetrakis(3,5-dihydroxyphenyl)porphyrin 4 and [G-2]Br; yield:
4
68%; UV–VIS, l/nm (e): 428 (534 000), 518 (6000), 560 (20 000),
600 (7000). d 8.96 (s, 8 H, b-H), 7.48 (s, 8 H, Ar-H), 7.17 (m,
H
160 H, Ph-H), 7.03 (s, 4 H, Ar-H), 6.67 (d, 16 H, Ar-H), 6.53
(d, 32 H, Ar-H), 6.43 (t, 8 H, Ar-H), 6.39 (t, 16 H, Ar-H), 5.06
(s, 16 H, Bn-H), 4.83 (s, 32 H, Bn-H), 4.78 (s, 64 H, Bn-H). d
160.08, 160.00, 157.75 (Ar C-O); 149.79 (a C); 139.25, 139.16
C
solutions in CDCl on a Bruker WM 300 (300 MHz) spec-
trometer with the solvent proton signal as standard: 7.24 ppm
(Ar C-CH ); 136.62 (Ph C-CH2); 128.40, 127.81, 127.40 (Ph
C-H); 120.67, 106.57, 106.25, 101.65, 101.52, 96.67 (Ar C-H);
69.89 (Ar/Ph-CH ). Mass spectrum (MALDI-TOF): m/z calc.
3
2
for 1H and 77.0 ppm for 13C. The following abbreviations are
used: Dendritic aldehydes are referred to as [G-n]CHO, where
n designates the dendrimer generation. Dendritic free-base
porphyrins are referred to as H [G-n] P, where n, again,
2
6621; found 6630 (Calc. for C
N 0.85; found C 78.98, H 5.96, N 0.50%).
H
N O Zn; C 79.09, H 5.54,
436 364
4 56
2
4
designates the dendrimer generation. Ar refers to the aromatic
repeat units within the dendrimer. Ph refers to the phenyl end-
groups, the surface, of the dendrimer. Bn refers to the benzylic
positions within the dendrimer. Porph refers to the two protons
Zn[G-4] P 1d. This was prepared as above from zinc
tetrakis(3,5-dihydroxyphenyl)porphyrin 4 and [G-3]Br; yield:
4
20%; UV–VIS, l/nm (e): 430 (463 000), 560 (21 000), 600
(7000). d 9.00 (s, 8 H, b-H), 7.48 (s, 8 H, Ar-H), 7.19 (m, 320
H
J. Mater. Chem., 1998, 8(3), 519–527
525

H, Ph-H), 6.67 (s, 4 H, Ar-H), 6.52–6.38 (overlapping reson-
ances, 168 H, Ar-H), 4.91 (s, 16 H, Bn-H), 4.75–4.69 (overlap-
ping resonances, 224 H, Bn-H). d 160.03, 159.90, 159.86,
filtrate was flash chromatographed through a silica column,
eluting with chloroform. The product fractions were collected
and evaporated to dryness. The residual solid was taken up in
minimal chloroform and precipitated into methanol, so the
product could be handled as a powder.
C
157.76 (Ar C-O); 149.68 (a C); 139.16, 139.08 (Ar C-CH );
136.62 (Ph C-CH ); 132.10 (b C); 128.40, 127.78, 127.45 (Ph
2
2
C-H); 106.21, 101.42 (Ar C-H); 69.80 (Ar/Ph-CH ). Mass
spectrum (MALDI-TOF): m/z calc. 13 413; found 13 437 (Calc.
2
H [G-1] P 3a. This was prepared from [G-1]CHO 2a. The
2
4
aldehyde and pyrrole were allowed to react for 1.5 h, and the
oxidation was carried out for 2 h; yield: 30%; UV–VIS, l/nm:
280 (dendrimer units); 424 (porphyrin Soret); 516, 550, 590,
for C
H 5.79, N 0.59%).
H
N O Zn; C 79.16, H 5.62, N 0.42; found C 79.24,
884 748
4 120
648 (porphyrin Q-region); d 8.95 (s, 8H, b-H), 7.39–7.51 (m,
40H, Ph-H), 7.60 (d, 8H, Ar-H), 7.17 (t, 4H, Ar-H), 5.31 (s,
General procedure for synthesis of dendritic aldehydes
H
A mixture of the appropriate dendritic bromide (2.00 equiv.),
3,5-dihydroxybenzaldehyde (1.00 equiv.), potassium carbonate
(3.00 equiv.), and 18-crown-6 (0.20 equiv.) was refluxed under
nitrogen in dry THF for 48 h. The reaction was allowed to
cool then evaporated to dryness under reduced pressure. The
residue was partitioned between water and methylene chloride,
and the aqueous layer was extracted with methylene chloride.
16H, Bn-H), −2.82 (s, 2H, porph-H); d 143.99 (a C), 127.63
C
(b C), 119.68 (meso C), 70.38 (CH O), 127.68, 128.07, 128.64,
2
136.79 (PhC). Mass spectrum (MALDI): m/z, calc. 1464;
found 1469.
H [G-2] P 3b. This was prepared from [G-2]CHO 2b. The
2
4
aldehyde and pyrrole were allowed to react for 1.5 h, and the
oxidation was caried out for 2 h; yield: 26%; UV–VIS l/nm:
280 (dendrimer units); 424 (porphyrin Soret); 516, 550, 590,
The combined organic layers were then dried (MgSO ) and
evaporated to dryness. The product was purified by flash
4
chromatography through a silica column, eluting with methyl-
ene chloride to give dendritic aldehyde as a colorless glass.
648 (porphyrin Q-region); d 8.87 (s, 8H, b-H), 7.21–7.39 (m,
80H, Ph-H), 7.49 (d, 8H, Ar-H), 7.05 (t, 4H, Ar-H), 6.76 (d,
16H, Ar-H), 6.59 (t, 8H, Ar-H), 5.15 (s, 16H, Bn-H), 4.98 (s,
H
[G-1]CHO 2a. 3,5-Dibenzyloxybenzaldehyde was pur-
chased from Aldrich and used directly.
32H, Bn-H), −2.89 (s, 2H, porph-H); d 141.45 (a C), 128.51
C
(b C), 119.43 (meso C), 70.38, 70.18 (CH O), 99.02, 161.14,
2
107.08, 139.14 (ArC), 127.44, 127.90, 128.48, 136.68 (PhC).
[G-2]CHO 2b. This was prepared from [G-1]Br; yield:
83%; n/cm−1 2873 [Ar(CO)MH st.], 1669 cm−1 [Ar(CNO)H
(d, 2 H, Ar-H), 6.81 (t, 1 H, Ar-H), 6.67 (d, 4 H, Ar-H), 6.59
Mass spectrum (MALDI): m/z, calc. 3162; found 3171.
st.]; d 9.87 (s, 1 H, ArCHO), 7.28–7.41 (m, 20 H, Ph-H), 7.05
H [G-3] P 3c. This was prepared from [G-3]CHO 2c. The
H
2
4
aldehyde and pyrrole were allowed to react for 10 h, and the
oxidation was run for 5 h; yield: 29%; UV–VIS, l/nm: 280
(dendrimer units); 424 (porphyrin Soret); 516, 550, 590, 648
(t, 2 H, Ar-H), 5.07 (s, 8 H, Bn-H), 5.03 (s, 4 H, Bn-H); d
C
70.4, 70.6 (CH O); 106.6, 107.7, 137.0, 160.6 (ArC); 102.0, 104.7,
2
136.7, 160.5 (ArC); 128.0, 128.4, 128.9, 139.2 (PhC); 198.6
(porphyrin Q-region); d 8.91 (s, 8H, b-H), 7.19–7.39 (m, 160H,
Ph-H), 7.51 (d, 8H, Ar-H), 7.08 (t, 4H, Ar-H), 6.72 (d, 16H,
H
(ArCHO). Mass spectrum (MALDI): m/z, calc. 743; found 767.
Ar-H), 6.60 (d, 32H, Ar-H), 6.56 (t, 8H, Ar-H), 6.49 (t, 16H,
Ar-H), 5.09 (s, 16H, Bn-H), 4.90 (s, 32H, Bn-H), 4.88 (s, 64H,
[G-3]CHO 2c. This was prepared from [G-2]Br; yield:
49%; n/cm−1 2873 [Ar(CO)MH st.], 1683 cm−1 [Ar(CNO)H
(d, 2 H, Ar-H), 6.84 (t, 1 H, Ar-H), 6.66 (d, 8 H, Ar-H), 6.65
Bn-H), −2.91 (s, 2H, porph-H); d 143.93 (a C), 128.53 (b C),
119.69 (meso C), 69.89, 69.88, 70.20 (CH O), 101.51, 101.53,
C
st.]; d 9.84 (s, 1 H, ArCHO), 7.29–7.42 (m, 40 H, Ph-H), 7.08
H
2
106.59, 106.23, 139.17, 139.14, 157.89, 160.03 (ArC), 127.49,
128.06, 128.53, 136.67 (PhC). Mass spectrum (MALDI) m/z,
calc. 6557; found 6577.
(d, 4 H, Ar-H), 6.55 (t, 4 H, Ar-H), 6.54 (t, 2 H, Ar-H), 5.00
(s, 16 H, Bn-H), 4.99 (s, 4 H, Bn-H), 4.95 (s, 8 H, Bn-H); d
C
69.9–70.2 (CH O); 106.4, 107.5, 136.7, 160.1 (ArC); 101.9, 103.9,
2
138.8, 160.1 (ArC); 127.7, 128.1, 128.7, 139.0 (PhC); 197.2
H [G-4] P 3d. Pyrrole (0.043 ml; 0.626 mmol) and 2d
(2.059 g; 0.626 mmol) were dissolved in dry CHCl and con-
densed in the presence of TFA (0.025 ml; 0.207 mmol). The
2
4
(ArCHO). Mass spectrum (MALDI): m/z, calc. 1592; found
1616.
3
solution was shielded from ambient light and stirred at room
temp. for 36 h. Then DDQ (0.177 g; 0.782 mmol) was added,
and the solution was allowed to stir at room temp. for an
additional 2 h. After cooling, the solution was evaporated to
dryness under reduced pressure and the residue was taken up
in a minimum amount of chloroform, then purified by flash
chromatography through a silica column eluting with chloro-
form. The product fractions were collected and evaporated to
dryness. The residual solid was taken up in a minimum amount
of chloroform and precipitated into methanol, so the product
could be handled as a powder; yield: 14%; UV–VIS, l/nm:
280 (dendrimer units); 424 (porphyrin Soret); 516, 550, 590,
[G-4]CHO 2d. This was prepared from [G-3]Br; yield:
56%. n/cm−1 2873 [Ar(CO)MH st.], 1694 cm−1 [Ar(CNO)H
(d, 2 H, Ar-H), 6.84 (t, 1 H, Ar-H), 6.64–6.67 (m, 28 H, Ar-H),
st.]; d 9.84 (s, 1H, ArCHO), 7.26–7.41 (m, 80 H, Ph-H), 7.08
H
6.54–6.56 (m, 14 H, Ar-H), 4.93, 4.96, 5.01 (s, 60 H, Bn-H); d
C
69.8–71.1 (CH O); 101.6, 106.4, 138.9, 160.1–160.3 (ArC); 127.5,
2
128.0, 128.6, 136.8 (PhC). Mass spectrum (MALDI) m/z, calc.
3290; found 3313.
General procedure for the preparation of porphyrin-core
dendrimers via Lindsey synthesis
648 (porphyrin Q-region); d 8.88 (s, 8 H, b-H), 7.38 (s, br, Ar-
H), 7.19–7.37 (m, Ph-H), 7.02 (s, br, Ar-H), 6.40–6.65 (m, Ar-
H), 4.98–4.85 (overlapping resonances, Bn-H), −2.91 (s, 2H,
porph-H); d 160.03, 159.90, 159.86, 157.76 (ArC-O); 149.68 (a
A solution of the appropriate dendritic aldehyde 2a–d (1.00
equiv.), freshly distilled pyrrole (1.00 equiv.), and 1 drop of
freshly distilled trifluoroacetic acid (TFA) was prepared in dry
methylene chloride whose volume was calculated so either
starting material had a concentration equal to 10−2 . The
reaction was shielded from ambient light and stirred at room
temp. for a period of time designated in the following text.
Then chloranil (8.00 equiv.) was added, and the reaction
mixture was warmed to reflux. This was stirred for a period
of time designated in the following text. The solution was
allowed to cool then evaporated to dryness under reduced
pressure. The residue was taken up in minimal chloroform,
and insoluble solids (i.e. excess chloranil) were removed. The
H
C
C); 139.16, 139.08 (ArC-CH ); 136.62 (Ph C-CH ); 132.10 (b
C); 128.40, 127.78, 127.45 (PhC-H); 106.21, 101.42 (ArC-H);
69.80 (Ar/Ph-CH ). Mass spectrum (MALDI-TOF): m/z, calc.
2
2
2
13 350 g mol−1; found 13 376.
General procedure for the metallation of free-base porphyrin-
core dendrimers
Introduction of zinc into dendrimers 3a–d was achieved by
dissolving the free-base porphyrin-core dendrimer (1.00 equiv.)
526
J. Mater. Chem., 1998, 8(3), 519–527

and Zn(OAc) Ω2H O (1.10 equiv.) in chloroform–methanol
saturated NaHCO , washed once with distilled water, then
dried over Na SO . The solvent was evaporated, and the
product was isolated as purple crystals which were dried under
2
2
3
4
(151) (100 ml) and heating the solution at reflux overnight.
The solvent was then evaporated, and the residual solids were
partitioned between water and CH Cl . The organic layer was
2
vacuum at 40 °C; yield: 100%; UV–VIS l/nm (e): 424 (242 000),
516 (14 000), 552 (5000), 592 (4000), 648 (2000); d
2
2
separated and dried (Na SO ). The solvent was evaporated to
2
4
H
afford a violet solid. The solid was redissolved in a minimum
amount of chloroform then precipitated into methanol, so the
compound could be handled as a powder. (Nb the characteriz-
ation of 1b–d is given earlier in this section.)
([2H DMSO, 2.49 ppm) 8.93 (s, 8 H, b-H), 7.05 (s, 8 H, Ar-H),
6
6.64 (s, 4 H, Ar-H), 3.81 (s, br, Ar-OH). d ([2H ]DMSO,
C
6
39.5 ppm) 156.54 (ArC-OH); 144.97 (ArC-porph); 127.50 (b C);
117.81 (meso C); 114.15, 102.50 (ArC-H). Mass spectrum
(MALDI-TOF): m/z calc. 743; found 753.
Zn[G-1] P 1a. This was prepared from H [G-1] P 3a;
yield: 100%; UV–VIS l/nm (e) 428 (568 000), 560 (22 000), 600
(7000). d 8.98 (s, 8 H, b-H), 7.48 (d, 8 H, Ar-H), 7.35 (m, 40
4
2
4
Financial support of this research by the National Science
Foundation (DMR-9641291) and by the MURI program of
AFOSR is acknowledged with thanks.
H
H, Ph-H), 7.02 (t, 4 H, Ar-H), 5.18 (s, 16 H, Bn-H). d 157.78
C
(ArC-O); 149.89 (a C); 144.63 (ArC-porph); 136.75 (PhC-CH );
2
131.97 (b C); 128.58, 128.01, 127.63 (Ph C-H); 120.67 (meso
References
C); 115.06, 96.67 (Ar C-H). Mass spectrum (MALDI-TOF):
m/z calc. 1527; found 1538 (Calc. for C H N O Zn; C 78.65,
H 5.02, N 3.67; found C 78.60, H 5.05, N 3.54%).
1
(a) G. R. Seely, Photochem. Photobiol., 1978, 2, 107; (b) R. Wagner,
J. Ruffing, B. Breakwell and J. Lindsey, T etrahedron. L ett., 1991,
32, (14), 1703; (c) R. Wagner, J. Lindsey, I. Turowska-Tyrk and
W. Scheidt, T etrahedron, 1994, 50, 11097.
100 76
4 8
Zinc tetrakis(3,5-dihydroxyphenyl)porphyrin 4. Tetrakis(3,5-
dihydroxyphenyl)porphyrin
2
3
J. Collman, J. Brauman, J. Fitzgerald, P. Hampton, Y. Naruta,
J. Sparapany and J. Ibers, J. Am. Chem. Soc., 1988, 110, 3477.
(a) S. Quici, S. Banfi and G. Pozzi, Gazz. Chim. Ital., 1993, 123, 597;
(b) C. Quintana, R. Assink and J. A. Shelnutt, J. Inorg. Chem., 1989,
28, 3421; (c) J. K. M. Sanders, Proc. Ind. Acad. Sci., Chem. Sci.,
1994, 106 (5), 983; (d) K. M. Faulkner, S. I. Liochev and
I. Fridovich, J. Biol. Chem., 1994, 269 (38), 23471; (e) T. Katsuki,
Kikan Kagaku Sosetsu, 1993, 19, 67; ( f ) D. R. Benson,
R. Valentekovich, S. W. Tam and F. Diederich, Helv. Chim. Acta,
1993, 76 (5), 2034; (g) H. L. Anderson, R. P. Bonar-Law,
L. G. Mackay, S. Nicholson and J. K. M. Sanders, NAT O ASI
Ser., Ser. C, 1992, 371 (Supramol. Chem.), 359; (h) D. Mansuy and
M. Fontecave, Biochem. Biophys. Res. Commun., 1982, 104 (4),
1651.
(a) F. R. Long, ‘Porphyrin Chemistry Advances’ Ann Arbor Science
Publishers Inc., Ann Arbor, 1979a; (b) F. Montanari and
L. Casella, ‘Metalloporphyrins Catalyzed Oxidations’, Kluwer
Academic Press, Boston, 1994; (c) R. A. Sheldon,
‘Metalloporphyrins in Catalytic Oxidations’, Dekker, New York,
1994.
(a) D. A. Tomalia, A. M. Naylor and W. A. Goddard III, Angew.
Chem., Int. Ed. Engl., 1990, 29, 138; (b) G. R. Newkome,
C. N. Moorefield and G. R. Baker, Aldrichim. Acta, 1992, 25, 31; (c)
B. I. Voit, Acta Polym., 1995, 46, 87; (d) E. M. M. de Brabander-
van den Berg, A. Nijenhuis, M. Mure, J. Keulen, R. Reintjens,
B. Vandenbooren, B. Bosman, R. de Raat, T. Frijns, S. v.d. Wal,
M. Castelijns, J. Put and E. W. Meijer, Macromol. Symp., 1994, 77,
51; (e) D. A. Tomalia, Adv. Mater., 1994, 7/8, 529.
(a) J. M. J. Fre´chet, Y. Jiang, C. J. Hawker and A. E. Philippides,
Preprints IUPAC Int. Symp. Functional Polym., Seoul, 1989, p. 19;
(b) C. J. Hawker and J. M. J. Fre´chet, J. Chem. Soc., Chem.
Commun., 1990, 1010; (c) J. M. J. Fre´chet, Science, 1994, 263, 1710.
(a) P. J. Dandliker, F. Diederich, M. Gross, C. B. Knobler,
A. Louati and E. M. Sanford, Angew. Chem. Int., Ed. Engl., 1994,
33, 1739; (b) J. P. Collman, L. Fu, A. Zingg and F. Diederich, Chem.
Commun., 1997, 193.
6 (700 mg, 0.942 mmol) and
Zn(OAc) Ω2H O (228 mg, 1.043 mmol) were dissolved in meth-
2
2
anol (20 ml). The solution was heated to reflux for 4 h, then
distilled water (60 ml) was added, and the methanol was
evaporated under vacuum. The turbid solution was placed in
the refrigerator overnight, and the purple crystalline product
was filtered and dried in vacuo at 40 °C; yield: 99%; UV–VIS,
l/nm (e): 428 (523 000), 560 (20 000), 600 (7000).
d
H
([2H ]DMSO, 2.49 ppm) 9.60 (s, 8 H, Ar-OH), 8.86 (s, 8 H, b-
6
H), 7.01 (d, 8 H, Ar-H), 6.62 (t, 4 H, Ar-H). d ([2H DMSO,
C
6
39.5 ppm) 156.21 (ArC-OH); 148.93 (a C); 144.46 (ArC-porph);
131.33 (b C); 120.23 (meso C); 114.16, 101.72 (ArC-H). Mass
spectrum (MALDI-TOF): m/z, calc. 806; found 814.
4
5
Tetrakis(3,5-dimethoxyphenyl)porphyrin 5. 3,5-dimethoxy-
benzaldehyde (1.500 g, 9.026 mmol) and freshly distilled pyr-
role (626 ml, 9.026 mmol) were dissolved in dry chloroform
(903 ml) under nitrogen atmosphere. After adding BF ΩOEt
3
2
(364 ml, 2.979 mmol) to the mixture, the solution was shielded
from ambient light and stirred at room temperature for 90 min.
After adding DDQ (1.537 g, 6.770 mmol), the mixture was
stirred for an additional 90 min, then triethylamine (415 ml,
2.979 mmol) was added to neutralize the acid. The solvent was
evaporated, and the residual solids were adsorbed onto silica
(20 ml). The crude product was purified by chromatography
through a 40 ml silica column eluting with a linear gradient of
solvent from pure hexane to pure CH Cl . After collecting the
6
7
8
2
2
product fractions and evaporating the solvent, the product
was obtained as purple crystals; yield: 52%; UV–VIS, l/nm (e)
424 (280 000), 516 (19 000), 548 (9000), 586 (9000), 648 (5000).
(a) R. Jin, T. Aida and S. Inoue, J. Chem. Soc., Chem. Commun.,
1993, 1260; (b) D. Jiang, R. Jin and T. Aida, Chem. Commun., 1996,
1523; (c) Y. Tomoyose, D. Jiang, R. Jin, T. Aida, T. Yamashita,
K. Horie, E. Yashima and Y. Okamoto, Macromolecules, 1996,
29, 5236.
d
([2H DMSO, 2.49 ppm) 8.95 (s, 8 H, b-H), 7.01 (s, 8 H, Ar-
H
6
H), 6.65 (s, 4 H, Ar-H), 3.33 (s, 24 H, ArO-CH ), −3.07 (s, 2
H, N-H); d ([2H DMSO, 39.5 ppm) 156.60 (ArC-OCH );
142.89 (ArC-porph); 131.12 (b C); 119.94 (meso C); 114.19,
3
C
6
3
102.28 (ArC-H); 48.64 (Ar-OCH ) (Calc. for C H N O ; C
73.05, H 5.42, N 6.55; found C 73.11, H 5.29, N 6.43%).
9
K. W. Pollak, J. W. Leon and J. M. J. Fre´chet, Polym. Mater. Sci.
Eng., 1995, 73, 137; K. W. Pollak, J. W. Leon and J. M. J. Fre´chet,
Chem. Mater., in the press.
3
52 46 4 8
10 P. Bhyrappa, J. K. Young, J. S. Moore and K. S. Suslick, J. Am.
Chem. Soc., 1996, 118, 5708.
11 C. J. Hawker and J. M. J. Fre´chet, J. Am. Chem. Soc., 1990, 112,
7638.
12 J. Lindsey, I. Scheirman, H. Hsu, P. Kearney and A. Marguerettaz,
J. Org. Chem., 1987, 52, 827.
13 R. J. Abraham, G. E. Hawkes, M. F. Hudson and K. M. Smith,
J. Chem. Soc., Perkin T rans. 2, 1975, 204.
Tetrakis(3,5-dihydroxyphenyl)porphyrin 6. Tetrakis(3,5-
dimethoxyphenyl)porphyrin 5 (740 mg, 0.866 mmol) was dis-
solved in dry CH Cl (20 ml) under nitrogen atmosphere. The
2
2
solution was cooled to 0 °C, then BBr (7.27 ml, 7.271 mmol,
1  in CH Cl ) was slowly added to the reaction. After
3
2
2
addition, the mixture was allowed to warm to room temp. as
it stirred overnight. Enough methanol was then added to
deactivate any unreacted BBr , distilled water (30 ml) was
added, and the mixture was stirred for 2 h. After evaporation
of the organic solvents, a green powder was filtered from the
water. It was dissolved in diethyl ether, washed twice with
14 (a) D. Dolphin, ‘T he Porphyrins vol. III’, Academic Press, NY 1978,
pp. 12–16; (b) J. E. Falk, ‘Porphyrins and Metalloporphyrins’,
Elsevier Publishing Company, NY, 1964, vol. 75–76, pp. 243–246.
3
Paper 7/05410F; Received 28th July, 1997
J. Mater. Chem., 1998, 8(3), 519–527
527